The present invention relates generally to an air-fuel ratio control system for an internal combustion engine and more particularly, to an air-fuel ratio control system which fulfills feedback control of an air-fuel ratio.
One of conventional air-fuel ratio control system for an internal combustion engine is disclosed, for example, in JP-A 60-240840.
With this conventional air-fuel ratio control system, an intake air flow Q and a rotating speed N of the engine are detected to calculate a basic fuel supply amount T.sub.P (=K.multidot.Q/N wherein K is a constant) corresponding to an amount of air inhaled in cylinders. This basic fuel supply amount T.sub.P is corrected by an engine temperature, etc., which is subjected to feedback correction in response to a signal derived from an air-fuel ratio sensor or oxygen sensor for sensing the air-fuel ratio of air-fuel mixture based on detection of an oxygen concentration in exhaust, and also correction by a battery voltage, etc., determining finally a fuel supply amount T.sub.I.
A drive pulse signal having a pulse width corresponding to the fuel supply amount T.sub.I thus determined is output at a predetermined timing, injecting and supplying a predetermined amount of fuel to the engine.
Air-fuel ratio feedback correction in response to a signal derived from the air-fuel ratio sensor is carried out to control the air-fuel ratio in the vicinity of a target air-fuel ratio or theoretical air-fuel ratio. Because a conversion efficiency or purification efficiency of a catalytic converter rhodium disposed in an exhaust system for oxygenating carbon monoxide (CO) and hydrocarbon (HC) in exhaust and reducing nitrogen oxides (N0.sub.X) therein for purification is determined to effectively function in an exhaust state upon theoretical air-fuel ratio combustion.
A proportional part and an integral part are determined in accordance with, for example, a deviation between the air-fuel ratio sensed by the air-fuel ratio sensor and the target air-fuel ratio, respectively. A value obtained by adding the proportional part and the integral part is multiplied, as a feedback correction factor ALPHA, by the basic fuel supply amount T.sub.P, controlling the air-fuel ratio in the vicinity of the theoretical air-fuel ratio.
With the prior art which fulfills such feedback control of the air-fuel ratio, the optimum values of the proportional part P and the integral part I vary according to engine operating conditions such as rotating speed, load, etc. Thus, some air-fuel ratio control systems allocate these parts in accordance with the engine operating conditions. In this case, without subdividing an operating area to be allocated, or adding an interpolating operation, a difference is produced between each required value and a set value, resulting in deteriorated accuracy. However, the above addition causes a problem that a microcomputer for carrying out control computing undergoes a heavy burden.
For varying the parts according to the engine operating conditions, the following methods are applicable: increasing an update amount per time as the rotating speed is higher by updating the integral part in synchronism with the rotating speed; determining, as the integral value, a value obtained by multiplying an integrated value of an update amount of a certain integration constant by a load such as fuel injection amount T.sub.P or T.sub.I so as to be increased as the load is greater; determining the integral part substantially as the inlet air flow Q is greater by combining the above two methods with each other. However, such methods are not always effective in determining the optimum value of the integral value, and thus further improvement of the control accuracy can be expected.
It is, therefore, an object of the present invention to provide a system for and method of controlling an air-fuel ratio in an internal combustion engine which allows feedback control of the air-fuel ratio with higher accuracy.